Conventional wellhead system includes a wellhead housing mounted on the upper end of a subsurface casing string extending into the well bore. A riser and blowout preventer (BOP) are then installed. During the drilling procedure, the BOP is installed above a wellhead housing (casing head) to provide pressure control as casing is installed, with each casing string having a hanger on its upper end for landing on a shoulder within the wellhead housing. Upon completion of this process, the BOP is replaced by a Christmas tree installed above the wellhead housing, with the tree having a valve to enable the oil or gas to be produced and directed into flow lines for transportation to a desired facility.
In accordance with a relatively recent development in this field, the conventional casing and tubing heads making up the Christmas tree are replaced by a horizontal tree which comprises a spool with a generally horizontal through port mounted above and in axial alignment with a horizontal through port in the wellhead housing. In this application, the hangers for the casing strings are supported one above the other within the bore of the wellhead housing, and the tubing hanger for the production or tubing string is supported in the bore of the spool to suspend the production string within the casing strings.
The vertical bore through the tubing hanger of a horizontal tree may be closed by a wire line tool to direct production fluid through aligned side ports (generally horizontal through ports) in the hanger and spool for recovery and delivery of production fluid to a suitable location. A redundant seal may be provided by a well cap installed in the tree above the tubing hanger, with the vertical bore aligned with that of the tubing hanger closed by a wire line plug to permit vertical access to the production tubing string upon removal of the plug.
The completion of a well with a horizontal tree conventionally includes an isolation/orientation sleeve which is installed within the tree bore in order to isolate and thus allow testing from the exterior of a metal seal between the adjacent ends of the wellhead housing and spool. A portion of the sleeve also carries a guide tube with an upwardly contoured guide surface positioned to cooperate with a lug on the tubing hanger to rotate the tubing hanger into a desired position as it is lowered onto a shoulder in the bore of the spool. This technique automatically aligns the side port in the hanger with the side port in the tree bore.
A BOP stack then is lowered onto the upper end of the spool with a bore in alignment with the spool bore, and a tubing hanger then lowered through the BOP and into the spool bore. A lug on the lower end of the hanger cooperates with the guide surface to orient the hanger into a proper landed position on a seat in the bore of the spool. A tree cap is then lowered into a landed position, following which the BOP may be removed to permit installation of a cover on the top of the tree.
More particularly, the complete drilling and completion operation involves lowering a wellhead housing at the upper end of a production casing string onto an outer housing at the upper end of a conductor casing surrounding the production casing string. The conductor casing and housing at its upper end conventionally have been lowered onto the ocean floor to suspend the production casing string within a well bore by means of a permanent base having guide posts, using cables extending to the surface.
The wellhead housing is lowered with the casing by a running tool on a drill pipe into the upper end of a BOP whose lower end is connected to the wellhead housing by means of a releasable connector, with the bores of the wellhead housing, the BOP and drill pipe in axial alignment. The BOP is normally of the ram type well known in the art and having a stack including at least one pipe ram and at least one blind ram. The upper outer diameter of the wellhead housing may have grooves thereabout to receive teeth of the releasable latch of the connector.
An outer casing hanger on the upper end of the casing may be lowered for landing onto a shoulder in the bore of the wellhead housing to suspend it within the conductor casing. An inner casing hanger in turn may be lowered into a landed position on the outer casing hanger to suspend an inner production casing within the casing. The hangers and the strings on which they are mounted may thus be lowered into the well on running strings through the riser and BOP bore.
A wear bushing may then be lowered through the riser and BOP for landing with its bore aligned with that of the casing hanger and extending upwardly within the bore of the wellhead housing so as to prevent wear on the seal at the upper bore of the wellhead housing during drilling of the production well. The lower portion of this bore is aligned with the bore of the upper casing hanger, while the upper end is enlarged to provide a tapered seat at its lower end. Each of the casing hangers and the wear bushings are releasably locked in place and the outer diameters of the hangers are sealed with respect to the bore of the wellhead housing.
At this stage, the wear bushing may be raised by a suitable running tool, and the BOP and connector at its lower end are released from the upper end of the wellhead housing and raised to the surface to enable the spool of a horizontal tree to be lowered on a running tool releasably connected to its upper end. The tree may then be guidably lowered onto the upper end of the wellhead housing by means of sleeves lowered over the guide posts. The tree may be releasably connected at its lower end to the upper end of the wellhead housing. Thus, the connector has latches on its lower end which move over the upper end of the housing for locking thereto with their bores aligned.
Upon landing of the horizontal tree on the wellhead housing, a lower end of an orientation sleeve in the tree moves into the upper enlarged end of the casing hanger to form a continuation of the bore therethrough. This lower end of the orientation sleeve carries a seal assembly for sealing between it and the upper enlarged bore of the casing hanger. A seal which was lowered with the upper casing hanger seals off the annulus between the casing hanger and well bore housing. A metal seal ring is carried by the tree to land upon and form a seal with a tapered seat on the inner diameter of the upper end of the bore of the housing so as to be energized as the tree is connected to the housing by a connector.
The upper portion of the bore through the horizontal tree may be enlarged to receive an elongated wear bushing which is lowered with the tree to land on an intermediate shoulder in the upper enlarged bore of the tree. The wear bushing has an inner diameter somewhat less than that of the bore of the tree so as to prevent wear on the seal surfaces of the tree bore as tools are lowered into and raised from the well bore during drilling of the hole to receive the tubing.
In accordance with one of its functions, the sleeve isolates the metal seal ring to permit the ring to be tested from the outside of the tree. In addition, the sleeve has an upwardly extending guide surface which cooperates with a lower guide surface on a tubing hanger to orient the tubing hanger into a desired rotational position as it is lowered into the tree bore. Thus, for horizontal trees of this type, a side port in the tubing hanger is aligned with side port in the tree to direct production fluid to a flow line.
In accordance with horizontal tree practices, a first plug is lowered on a wire line and locked within the upper end of the hanger bore above its side port, and a tree cap that may contain another wire line plug is lowered into and locked within the bore of the tree. The BOP is then removed and replaced by a protective cover which is axially aligned and rotationally oriented into a position over the upper end of the tree.
The conventional procedure for installing a horizontal tree on a subsea wellhead involves the installation and the removal, and the subsequent reinstallation, of the BOP stack. U.S. Pat. Nos. 5,465,794; 5,544,707; 5,555,935; 6,062,314 and 6,039,119 disclose different equipment for horizontal tree applications. Each such installation is time consuming and expensive.
The present invention provides an improved horizontal spool tree wellhead system and method in which only one installation of the BOP stack is required. The disadvantages of the prior art are overcome by the present invention, and improved horizontal spool tree wellhead system and method are hereinafter disclosed which does not require repeated installation and removal of the BOP stack.